1. Field of the Invention
This invention relates to a multi-metallic bulk catalyst with layered structure.
This invention also relates to a method for the preparation of the above catalyst.
This invention also relates to use of the above catalyst in ultra-deep hydrodesulfurization of sulfur-containing compounds.
2. Description of the Related Art
As the requirements of the environmental regulations in each country on the sulfur content in fuel oil is stringent increasely, and the high-sulfur crude oil is increased around the world year after year, the development of ultra-deep hydrodesulfurization catalysts with high performance has been a challenging subject worldwide. It is well-known that SOx from the burning of organic sulfur-containing compounds in fuel oils not only can cause acid rain, but also can poison irreversibly the three-way catalysts in the tail gas cleanup systems of engines. Therefore, it has been receiving much attention. For this reason, all countries in the world have enacted much more stringent sulfur specifications of diesel. China will implement the sulfur specifications equal to Europe IV emission standard (<50 ppm) in Jul. 1, 2010 (Big cities in China, such as Beijing and Shanghai, will advance two years to implement the standard).
Currently, the hydrodesulfurization catalysts commonly used in industry are Co—Mo/Al2O3, Ni—Mo—P/Al2O3, Ni—W—B/Al2O3, Ni—Co—Mo/Al2O3 and Co—W/Al2O3, and the like. However, as the standard defined for sulfur content is becoming much higher, the activities of these catalysts cannot meet the demand of ultra-deep desulfurization. Thus, there is an urgent need for improving the hydrodesulfurization activity for the catalyst. The adjustment of operation conditions and the use of a new reactor both need a big amount of investment cost. In comparison, the development of a new hydrodesulfurization catalyst based on the existing operating conditions which can be used in an existing production unit is a much more economical and feasible method.
The sulfur-containing compounds contained mainly in diesel are thiols, thioethers, thiophenes and derivatives thereof, benzothiophenes and derivatives thereof, dibenzothiophenes and derivatives thereof, in which 4,6-DMDBT is the most difficult sulfur—containing compound to be removed through hydrodesulfurization means. The hydrodesulfurization activity of a traditional supported catalyst is hard to be improved generally by a large margin because the support itself therein has only limited catalytic effect, wherein the catalytic activity could only be improved by increasing the contact area between the support and reactants or using the synergistic effect between the support and the active species. However, a multi-metallic bulk catalyst, in other words, an unsupported multi-metallic catalyst is a catalyst with very high hydrodesulfurization activity because it has different kinds of active species and much more active sites than those of the supported catalyst. Among other, a bulk catalyst having a metallic composition of NiMoW, reported in the recent references and patents, has attracted much more attention due to having extremely high hydrodesulfurization activity.
In the U.S. Pat. Nos. 6,299,760, 6,156,695, 6,783,663, 6,712,955 and 6,758,963 and the like, the preparation methods and uses of novel NiMoW bulk catalysts were reported. Moreover, such a novel NiMoW catalyst exhibits a hydrodesulfurization activity which is about 3 times higher than that of other commercial reference catalyst. In all the preparation processes of the catalysts, ammonia was used as a complexing agent which was complexed with raw material, Ni2+, followed by a slowly heating, wherein the Nickel-ammonia complex was decomposed slowly to Ni2+, which was reacted with molybdenum and tungsten in the solution to form a NiMoW catalyst precursor. Upon calcination and sulfidation, the NiMoW catalyst was formed. The preparation method had a disadvantage that the use of concentrated aqueous ammonia during the synthesis could cause pollution on the environment.
It was reported by G. Alonso-Nunez et. al, in references (Applied Catalysis A: General, 304 (2006), 124-130; Applied Catalysis A: General, 302 (2006) 177-184; Catalysis Letters, 99 (2005) 65-71) that a NiMoW catalyst was synthesized by using different materials and sulfidation reagents. The catalyst obtained via synthesis by them had a special scaly shape and may have a relatively high surface area. However, the production cost for the catalyst was increased because of the complex synthesis method they used and expensive starting materials. Thus, it was hard to realize the industrialization thereof.
In the Chinese Patent No. 1339985A, a preparation method and use of NiMoW catalyst was disclosed. In the patent, the catalyst was obtained by reacting a tungsten salt, a molybdenum salt and basic nickel carbonate in an aqueous solution, followed by a sulfidation process. During the reaction, at least part of the metallic components was ensured to be existed in the solid form. In the patent, part of the raw materials used is basic nickel carbonate, which is insoluble in water, the essential reaction during the syntheses is an exchange reaction between the ions and solid. Thus, it was hard to prepare catalyst particles with small grains.
It is obviously found from the prior art that the preparation methods of the multi-metal bulk catalysts have the disadvantages of:
(1) the raw materials used are not friendly on the environment;
(2) The preparation costs of the catalysts are relatively high; and
(3) The activities of the catalysts needs to be further improved.
Thus, it is greatly desired to develop a multi-metallic bulk hydrodesulfurization catalyst and a method for the preparation thereof, wherein the catalyst has super high activity which starting materials are facile, and the catalyst are friendly on the environment and the cost thereof are relative.